Optical systems that can maintain a constant-magnification image of a viewed object, as a distance of the viewed object from the optical system varies, are useful. Such optical systems have advantages in imaging applications that require recognition or measurement of an object over a range of object distances.
One type of optical system that provides such a capability includes two or more moving optical elements that have a physical distance between them that is mechanically changed to provide an image of the viewed object. For example, U.S. Pat. No. 5,095,388 describes a zoom lens system for photographing objects, located within a broad range of distance, at definite magnification levels. This system is complex, and includes a first and second lens unit, and a first and second mechanism. The first mechanism varies the focal length of the first lens unit and the magnification of the second lens unit, while the second mechanism varies a space between the two lens units. A constant magnification level is achieved by having the first mechanism physically move the lens units along the optical axis in such a manner so as to maintain a constant ratio between the focal length of the first lens unit and the lateral magnification of the second lens unit.
Another type of optical system that can provide a constant magnification over a range of object distances is a fixed focal length telecentric system. However, such telecentric optical systems have drawbacks. Telecentric optical systems generally include at least an objective lens and an aperture stop. The objective lens and an aperture stop are arranged along an optical axis with the aperture stop being located a distance of one focal length of the objective lens away from the objective lens. Although telecentric lens systems provide nominally constant magnification over a range of object distances away from the system, the image that they provide does not remain focused outside of their limited depth of focus. Thus, their utility is limited.